Conventionally, there are opening devices for straws formed in the thermoplastics lid of the paper container filled up with liquid food, such as cow's milk and soft drinks. In the opening device for straws, as for the packaging material of the liquid food container, a paper substrate is covered by an outer layer and an inner layer of thermoplastics, a straw hole is covered with thermoplastics and a straw hole sealing part is formed. The straw hole sealing part is formed by injection molding. In the straw hole sealing part, it is required for the straw to penetrate easily without sliding, and for the straw hole sealing part to be soft and to keep the tip of the straw from bending, when sticking the straw.
Therefore, a thin-walled groove portion is formed in the straw hole sealing part to such an extent that sealing performance is not damaged. (Reference patent documents 1) Patent documents 1: JP,2002-362548,A